A hydraulic drive system for a work machine including a recovery circuit for reutilizing (recovering) hydraulic fluid discharged from a boom cylinder by own weight falling of a boom for driving an arm cylinder is known, and an example of the hydraulic drive system is disclosed in Patent Document 1. In the hydraulic drive system of Patent Document 1, when discharged fluid from a boom cylinder is to be recovered to an arm cylinder, the delivery flow rate of a hydraulic pump that supplies hydraulic fluid to the arm cylinder is decreased as much to achieve improvement of the fuel cost of an engine.